Handbook for Sound Engineers

Digital Audio Interfacing and Networking 1505

of these bundles. If no receiver is set for this bundle

number the bundle will not be transmitted.

39.12.2 Protocol

CobraNet® operates at the data link layer (OSI Level
2). It uses three distinct packet types, all of which are
identified in the Ethernet packet by the unique protocol

identifier (8819 hex) assigned by the IEEE to Peak
Audio. CobraNet® is a local area network (LAN) tech-

nology and does not utilize Internet Protocol (IP), which
is most important in wide area networks (WAN).

39.12.3 Beat Packet

Beat packets are sent with the multicast destination

MAC address 01:60:2B:FF:FF:00. They contain the
clock, network operating parameters, and transmission
permissions. The beat packet is sent by the Conductor

and indicates the start of the isochronous cycle. Because
the beat packet carries the clock for the network, it is

sensitive to delay variations in its delivery to all the
other CobraNet® interfaces. Failure to meet the delay

variation specification can keep the other devices from
locking their local clocks to the master clock in the
Conductor. The beat packet is usually small on the order

of 100 bytes, but grows with the number of active
bundles.

39.12.4 Isochronous Data Packet

One isochronous data packet is transmitted for each

bundle each isochronous cycle, and carries the audio

data. It can be addressed to either unicast or multicast

destination addresses depending on the bundle number.

Since the CobraNet® interfaces buffer the data, out of

order delivery within an isochronous cycle is accept-

able. To reduce the impact of the Ethernet packet struc-

ture overhead on the total bandwidth consumed, data

packets are usually large on the order of 1000 bytes.

39.12.5 Reservation Packet

Reservation packets are sent with the multicast destina-

tion MAC address 01:60:2B:FF:FF:01. CobraNet®

devices usually send a reservation packet once per

second. This packet is never large.

39.12.6 Timing and Performance

In order for CobraNet® to provide real-time audio

delivery, certain maximum delay and delay variation

requirements must be put on the performance of the

Ethernet network, Fig. 39-55.

If the network loses a beat packet it will cause an

interruption in proper operation of the entire

CobraNet® network. If an isochronous data packet is

lost, a 1 ms dropout will occur only in the audio

carried by that particular bundle. A single such dropout

may be inaudible or may make a “tick” in the audio.

Large numbers of dropouts may sound like distortion.

(^1) » 3
Parameter Minimum Maximum Typical Comments
Isochronous Cycle Interval 1333 μs Future CobraNet® revisions may allow other cycle interval options.
Beat Packet Length 5.12 μs 121.4 μs d 10 μs Beat packet grows as bundle count increases.
Data Packet Length 5.12 μs 121.4 μs 100 μs Size dependent on audio resolution and number of audio channels carried
in bundle.
Reservation Packet Length 5.12 μs 121.4 μs 10 μs
Inter-Packet Spacing 0.96 μs5 μs
Beat Packet Delay Variation 0 μs 250 μs Normal delay distribution assumed.
Forwarding Delay 0 μs 400 μs Assume maximal packet length when calculating store and forward delay
(if applicable). Includes delay variation, i.e., 750 μs forwarding delay +
250 μs maximal positive excursion due to delay variation = 1000 μs. A
higher forwarding delay can be tolerated on networks with small delay
variation.
If the forwarding delay specification is exceeded, additional delay is auto-
matically added to the audio in increments of 64 sample periods ( 11 e 3 ms).
Figure 39-55. CobraNet“ packet timing and performance requirements for the Ethernet network. Make sure your Ethernet
switch vendor will guarantee that their switches in the configuration you propose will meet the above delay variation and
forwarding delay specifications.